Optical amplification is one of the enabling technologies in Wavelength Division Multiplexing (WDM) optical communication systems. Among optical amplifiers in current optical communication systems, the Erbium Doped Fiber Amplifier (EDFA) is the most commonly used. The EDFA gain medium is a silica fiber doped with Erbium ions, which is excited optically by a diode laser, typically at wavelengths 980 nm or 1480 nm. Due to the atomic properties of the Erbium ions in the silica fiber, the gain obtained at each of the wavelengths in an operating range, for example the C-band (1529-1563 nm) is different, and thus signals that enter with the same power into the amplifier can exit with power differences that can reach a few decibels [xe2x80x9cOptical Fiber Communication Systemsxe2x80x9d, L. Kozovsky, et. al., Artech House, 1996, p. 578-584]. For the reasons mentioned above, a gain flattening filter (GFF) is usually applied in order to achieve power equalization of the different wavelengths at EDFA output (Charplevy et al., U.S. Pat. No. 5,225,922; M. Tachibana et al. xe2x80x9cErbium doped fiber amplification with flattened gain spectrumxe2x80x9d IEEE Photonics Technology Letters, Vol. 3, pp118-120, 1991). Usually GFFs are passive filers (based on technologies such as Bragg gratings, long period gratings and thin film technology), which perform gain equalization for a certain EDFA gain. If an EDFA that is based on a passive GFF is operated at a different gain than the one designated by its manufacturer, its output is not gain-flattened any more.
As advanced optical networks are becoming dynamic and with complicated topologies (e.g. Mesh topology vs. Point-to-Point), the need to operate an EDFA with large dynamic gain range rises. Many technologies have been proposed for transforming the EDFA into a device with a large dynamic range. Some are based on dynamic filters positioned at the amplifier""s output or at its mid-stage (for a dual-stage amplifier). Such filters include acousto-optic tunable filters (e.g. Pearson, U.S. Pat. No. 5,514,413 and Olshansky, U.S. Pat. No. 5,276,543), liquid crystal filters (Kuang-Yi Wu, U.S. Pat. No. 5,963,291), and Mach-Zehnder filters (Miller, U.S. Pat. No. 5,351,325, Ranalli et. al., xe2x80x9cPlanar tapped delay line based, actively configurable gain-flattening filterxe2x80x9d, ECOC 2000, Vol. 3, p. 21). All such filters have a high excess loss, and require an optical spectrum-analyzing unit for control and operation. These solutions render the EDFA a highly expensive and energy inefficient unit, the latter because of the tunable filter high excess loss.
Another common way to achieve dynamic gain equalization is by using two amplifier stages with opposite gain tilts [Yadlowsky, U.S. Pat. No. 6,215,581B]. Opposite tilt signs are achieved by differentiating the optical pump level for each of the stages. However, it is well known that in this case the dynamic gain equalization range, within a specific flattening tolerance, is limited. Another applicable technique for dynamic gain equalization uses a variable optical attenuator (VOA) inserted in front of the amplifier (Sugaya, U.S. Pat. No. 5,812,710) or between the amplifier""s stages (Taylor, U.S. Pat. No. 6,049,413), to lower the signal power and accommodate the need for optimized gain for power equalization. However, it is well known to those skilled in the art that this technique wastes energy, and degrades the amplifier""s signal to noise characteristics. In order to improve the noise figure (NF) of an EDFA with a mid-stage VOA, Tomofuji et al. (EP 1,094,624A2) suggested constructing the first amplifying stage from two optical amplifiers and a VOA inserted between them. Yadlowsky, in U.S. Pat. No. 6,215,581B further suggested using both a VOA and a GFF, and separating the VOA from the GFF with an amplifying stage. Though this configuration improves significantly the noise figure of a large dynamic gain EDFA, adding a gain element between the VOA and GFF wastes energy, and either requires an additional pump source or splitting one pump source into two pump lines, each connected to a gain element before and after the VOA. Since each pump line is independent, this technique limits also the dynamic range of the gain that the amplifier can be operated in.
There is thus a widely recognized need for, and it would be highly advantageous to have a highly efficient EDFA having a high dynamic gain range and a low NF throughout the whole gain range over a wide spectral span. Therefore, it is a primary object of the present invention to provide a technique based on the use of a VOA and on the saturation properties of the erbium doped fiber (EDF) (which controls the pump-to-signal conversion efficiencyxe2x80x94see for example xe2x80x9cErbium-doped Fiber Amplifiersxe2x80x9d, P. C. Becker, et al., p. 156-161, Academic Press, 1999) to achieve within one stage of the EDFA a maximal gain range with optimal power efficiency and low noise figure.
The present invention is of a rare-earth doped fiber amplifier, specifically an EDFA, with a high dynamic gain range. The EDFA may contain a single gain stage or multiple gain stages, with an optional gain flattening filter inserted between successive stages, for achieving spectrally flattened signals at the EDFA""s output. The dynamic gain range is achieved by an interplay between the action of a VOA positioned between two EDF gain sections of one of the EDFA stages, and the pump energy absorption mechanisms at each gain section, which are dominated by the saturation characteristics of the EDF comprising each of the gain sections before and after the variable attenuator. Hereinafter, this EDFA stage is called the Dynamic Gain (xe2x80x9cDGxe2x80x9d) stage.
In the DG stage, the same pump is feeding both EDF gain sections in a way that the residual pump power coming out of the first EDF section feeds the second section. In contrast with prior art, the residual power pumping the second section is typically xe2x80x9cpassivexe2x80x9d in the sense that it is mainly determined by the energy absorption in the first section, and not actively by the sole pump. The VOA located between the two EDF gain sections of the DG stage affects only the output signals of the first section. A main object of the present invention is to utilize the VOA and the pump absorption relations between the two EDF sections in a way that renders the amplification of the DG stage more efficient, in terms of pump signal effective use and NF reduction, while enlarging the dynamic gain range in which the amplifier maintains a flattened output, over a wide spectral range.
According to one embodiment of the present invention there is provided an apparatus for amplifying a plurality of optical signals having different wavelengths, the optical signals entering at an input port and exiting at an output port of the apparatus, the apparatus comprising two, first and second fiber gain sections, each of the sections having a section input port and a section output port, a variable optical attenuator inserted between, and in optical communication with, the two fiber gain sections, a pump coupled to both the gain sections, the pump producing a main pump signal used to pump the first gain section, and a residual pump signal used to pump the second gain section, and means to control the attenuation of the variable optical attenuator, whereby the combined action of the two fiber gain sections and the variable optical attenuator provides dynamic gain equalization of the optical signals over a wide spectral range.
According to further features in the apparatus of the present invention there is provided a first coupler for coupling the input optical signals and the main pump signal into the first gain section, the coupled signals leaving the first gain section at its output port as first output optical signals, a second coupler for decoupling the first output signals into a residual pump power signal and into first amplified optical signals, the first amplified signals being input to the variable optical attenuator to obtain first attenuated optical signals, and the residual pump power signal being routed around the variable optical attenuator, and a third coupler for coupling the first attenuated signals and the residual pump power signal into the second fiber gain section, the second gain section outputting dynamically gain equalized amplifier output signals at the amplifier output port. Preferably, the fiber sections are made of Erbium doped fiber
According to another embodiment of the present invention there is provided a dual-stage apparatus for amplifying a plurality of optical signals having different wavelengths, comprising an amplification stage for providing first amplified signals, the amplification stage including two, first and second fiber gain sections, each of the sections having a section input port and a section output port, a variable optical attenuator inserted between and in optical communication with the two fiber gain sections, a pump coupled to both the gain sections, the pump producing a main pump signal used to pump the first gain section, and a residual pump signal used to pump the second gain section, the residual pump signal determined by energy absorption mechanisms in the first gain section, and means to control the attenuation of the variable optical attenuator and a booster stage in optical communication with the amplification stage for receiving the first amplified signals and for outputting high power dynamically gain equalized output signals. Preferably, the fiber sections are made of Erbium doped fiber.
According to the present invention there is further provided a method for dynamically gain equalizing optical signals having an input power level, comprising: providing an amplification stage having two, first and second fiber gain sections, providing a variable optical attenuator optically connected between the two fiber gain sections, using a single pump to pump the first fiber gain section and obtain first section amplified signals correlated with a residual pump power signal, attenuating the first section amplified signals in the attenuator to obtain first attenuated signals, and amplifying the attenuated signals in the second gain section while pumping the second gain section with the residual pump signal, thereby obtaining dynamically gain equalized signals. Preferably, the fiber sections are made of Erbium doped fiber
According to additional features in the method of the present invention, the residual pump power signal is determined by the absorption properties of the Er-fiber comprising the first gain section and by the input power level of the optical signals entering the first section.
The present invention successfully addresses the shortcomings of the presently known configurations by providing a method and apparatus for a highly efficient, high performance Er-fiber optical amplifier that employs one pump and a variable attenuator to achieve dynamic gain equalization over a large dynamic range.